This invention relates to a form of dry planography which might be referred to as "silicone rubber planography". More particularly, it relates to methods of making new pre-sensitized plates usable in such dry planography, developing them for use on a dry planographic offset or direct printing press and the new plates themselves.
Dry silicone rubber planography is an improvement or a simpliciation of conventional offset lithography. In conventional offset lithography an aqueous fountain solution is applied to the background area of the printing plate. This fountain solution wets the hydrophilic background areas of the plate rendering them non-ink receptive. The oleophilic image areas of the plate are not wet by the aqueous fountain solution and are, therefore, inked by the inking rollers. The ink on the image is next printed upon a rubber blanket from which it is "offset" or transferred to the final printing substrate usually a sheet of paper. The process is repeated for each of as many prints as are desired. Lithography gives excellent results; but the process requires complex printing presses as well as a considerably degree of expertise in their operation. One of the major problems encountered during lithographic printing is caused by the fountain solution which must be carefully controlled; too little fountain solution causes the background areas to tone and too much causes emulsification of the ink. Elimination of the fountain solution would decrease the amount of skill required to operate the presses as well as allowing the press design to be simplified considerably. The present invention is directed to a presensitized lithographic plate that eliminates the fountain solution and to a method of producing that plate.
The most common method of presensitizing photo-lithographic printing plates is to wash the surface of the plate with an aqueous solution of diazonium salt which, upon drying, leaves a thin layer of the diazonium compound on the surface of the plate. When exposed to actinic light through a negative transparency, the diazonium compound in the exposed image areas is converted to an aqueous insoluble oleophilic compound which adheres to the surface of the plate while the unexposed diazo remains soluble. Subsequent washing of the exposed plate with an aqueous developer solution removes the unexposed and still soluble diazo compound from the background areas, but leaves the insoluble oleophilic image firmly attached. The image thus produced accepts ink from the inking rollers.
In dry or silicone planography, the background areas of the plate are cured elastic silicone rubber which when dry without being wetted by water or other liquid, will not accept printing ink from a rotating printing roller passed thereover.
French Pat. No. 1,475,466 discloses an excellent diazo-sensitized plate capable of being imaged by exposure to light through a positive transparency. In this case the diazo image forming compound lies beneath the silicone rubber layer. In the exposed background areas the diazo compound becomes insoluble, while in the non-exposed image areas it remains soluble. When the exposed plate is developed by scrubbing with developer solution the still soluble diazo in the image area is loosened and removed along with the silicone rubber overlaying that area. Considerable attention to detail is required in removing the cured silicone rubber to lay bare the underlying ink-receptive image area, but when carefully done an excellent long-run plate results. The removal of cured silicone rubber is usually difficult, particularly when the film is more than a fraction of a mil in thickness and the use of films thicker than a fraction of a mil is generally desired. In many cases it would be preferably to use a thicker film of silicone if it were not so difficult to remove the silicone rubber in the image areas; for some reason not now apparent, thicker silicone rubber layers seem less prone than very thin layers to becoming "toned-" by repeated passage of an inking roller thereover. It is, therefore, desirable to produce a master which does not require the use of an undesirably thin layer of silicone rubber.
In addition, it is desirable to produce a presensitized plate where the image is firmly bonded to the silicone but the plate can be developed without removing portions of silicone or otherwise damaging the surface of the silicone layer. Production of such a master or plate entails affixing an ink-receptive image to the silicone surface. This is very difficult to accomplish, however, because the very properties of the silicone rubber which prevent ink from adhering to its surface also act to prevent the imaging material from firmly adhering to the surface of the silicone material. Thus, the procedures previously utilized to sensitize lithographic printing plates are not suitable for sensitizing the silicone rubber coated dry planographic plates. It has been found that neither aqueous solutions of diazonium sensitizers nor solutions in the usual organic solvents adhere as well as desired to the surface of silicone rubber for the silicone is not wet by such solutions and the sensitizer materials do not adhere to the silicone rubber.
It is possible to sensitize a dry silicone rubber surface as disclosed by Harry F. Gipe in pending U.S. Patent Application Ser. No. 706,286, filed Feb. 19, 1968 now U.S. Pat. No. 3,677,178, by rubbing dry powdered water-soluble diazonium salt into the silicone rubber surface. The plate so prepared can be exposed through an image bearing transparency and washed with water to remove the unexposed diazo compound in the background areas leaving the exposed reacted diazo material in the image areas. The exposed image areas accept printing ink and readily transfer the ink to the printing surface. Under the best conditions, however, only a few hundred copies can be printed before the image fails. Furthermore, such a method does not lend itself to modern high speed mass production methods; therefore, even for short run plates the method is not economically advantageous. The method of the present invention is directed to a method of applying the photoresponsive sensitizer material which results not only in good adhesion between the photo-responsive material and the silicone rubber background area but this method also lends itself to use in modern high speed production facilities. Furthermore, since developing the plate of the present invention does not entail removing part of the silicone layer, the silicone layer can be as thick as desired without adversely affecting developing characteristics or plate life.